Laser heterodyne radiometry is a technique for detecting weak absorption signals that was adapted from radio receiver technology. In a radio receiver, a weak input signal from a radio antenna is mixed with a stronger local oscillator signal. The mixed signal (beat note, or intermediate frequency (IF)) has a frequency equal to the difference between the input signal and the local oscillator. The intermediate frequency is amplified and sent to a detector that extracts the audio from the signal.
Since approximately 1971, laser heterodyne radiometers (LHR) have been used for atmospheric studies. The laser heterodyne receiver was originally developed for Earth studies and has demonstrated measurement of atmospheric ozone concentration profiles from the ground. A LHR is generally able to make measurements with an ultrahigh spectral resolution over a narrow range. In a laser heterodyne radiometer, the weak input signal is light that has undergone absorption by a trace gas. The local oscillator (LO) is a laser at a near-by frequency to the weak input signal. The two light waves are superimposed in a beam splitter or single mode fiber coupler, mixed in the high speed detector and the RF beat signal is extracted. Changes in the concentration of the trace gas are realized through analyzing changes in the beat frequency amplitude. The heterodyne method generally down-converts the received signal to a lower, intermediate frequency (IF) signal in order to process it more easily
Satellite instruments that measure carbon cycle gases in the atmospheric column need comparable ground validation measurements. The only network that currently measures carbon dioxide (CO2) and methane (CH4) in the atmospheric column is the Total Carbon Column Observing Network (TCCON). The TCCON is a network of ground-based Fourier Transform Spectrometers that record direct solar spectra in the near-infrared spectral region. For these spectra, accurate and precise column-averaged abundance of CO2, CH4, N2O, HF, H2O and HDO are retrieved. Only two of the TCCON operational sites are in the United States. TCCON data is used for validation of GOSAT data, and will be used for OCO 2 validation.
The existing network of ground Fourier Transform Spectrometer instruments making column measurements is sparse because cost and size limit their viability as a mass-produced ground network instrument. While the Fourier Transform Spectrometers of the TCCON network can measure the largest range of trace gases, the TCCON network is severely limited due to the high cost and extreme size of these instruments. Typically, these instruments occupy small buildings and require personnel for operation. It would be advantageous to provide a significantly small autonomous instrument that can be incorporated into the Aerosol Robotic Network (AERONET) program's much larger global network. AERONET is a globally distributed network of more than 450 land based aerosol sensing instruments (autonomous sun photometers).
Accordingly, it would be desirable to provide a laser heterodyne radiometer system that addresses at least some of the problems identified above.